1. Field of the Invention
This invention relates to a microporous, polyvinylidene difluoride (PVDF) membrane. More specifically, the invention relates to a microporous PVDF/support composite membrane and to the method for preparing such membrane. In particular, the invention relates to a microporous PVDF membrane which is laminated by a phase-inversion process to another porous membrane without the use of glue or binders, such that the exposed surface of the PVDF membrane is smooth and contains a multiplicity of discrete pore openings, having submicron diameters.
2. Description of the Prior Art
Microporous membranes are commonly used to separate components from a solution, to remove particulates from a liquid or gaseous medium or to transmit substances in a controlled manner. Such membranes find a variety of applications in diverse areas of technology, including, for example, recovery of electroplating metals, blood dialysis, maintenance of cell cultures, food processing, biosensors, sensing electrodes, purification of bioprocessing products, separators for battery compartments, sterilization of solutions, blood plasma processing by electrodialysis, artificial organ construction, protein separation, purification of natural flavors and fragrances, devices for controlled release of drugs, industrial waste treatment and uses in various other areas of technology. The wide variety of processes and conditions in which microporous membranes are used indicates a need for membranes with a wide range of physical, chemical and structural features.
The PVDF polymer has features, in addition to its very good membrane forming ability, that can accommodate a wide variety of operating conditions due to its chemical and biological inertness. Also, PVDF membranes are structurally useful at temperatures ranging from at least -150.degree. C. to 140.degree. C. Also, PVDF is non-toxic. Thus, PVDF is an outstanding material for microporous filtration or barrier membranes.
One of the most useful and common methods of forming microporous membranes from organic polymers is the phaseinversion method. In the case of PVDF, the application of the phase inversion procedure is complicated by the tendency of the gel to shrink and distort in the course of its formation and then to shrink and distort further upon drying to form a membrane.
Shrinkage and distortion of PVDF membranes can be controlled by physically restraining the gel during formation, however, physical restraint of shrinkage is technically complicated. Shrinkage is particularly problematic when, as required within this invention, a highly aqueous gelling medium is used, resulting in a distorted, shrivelled structure rather than a smooth, undisturbed membrane. Several phase inversion methods for producing undistorted microporous PVDF membranes without the need to restrain the gel are known, however, membranes formed by these methods have non-discrete pore openings embedded in a disturbed, sponge-like surface structure.
U.S. Pat. No. 3,642,668 to Bailey, et al. describes the use of alcohol as a gelling medium in a phase-inversion procedure to form PVDF membranes. In alcohol, shrinkage is greatly reduced, hence there is no need to physically restrain the gel during formation and sheets of PVDF membrane can be formed by extrusion of the PVDF solution through a slotted die directly into a methanol gelling bath.
In U.S. Pat. No. 4,203,848 to Grandine, acetone is used as a gelling medium. Mixtures of acetone and water, containing at least 50% acetone are used to prevent gel shrinkage during formation of a non-supported PVDF membrane.
U.S. Pat. No. 4,806,291 to Susa also discloses the use of alcohol as a gelation medium wherein the PVDF casting solution includes ammonia salts as pore-forming agents. The PVDF solution is cast upon a glass plate or upon a fibrous or porous supporting structure.
U.S. Pat. No. 4,384,047 to Benzinger describes a phase - inversion method for non-supported PVDF membrane formation where restraint during gel formation is unnecessary in order to obtain smooth, wrinkle-free membranes. Triethylphosphate is the solvent used for the PVDF and inclusion is required of any of the nonsolvents glycerol, ethylene glycol or phosphoric acid to act as pore-forming agents. Gelation is achieved in ice water.
In British Patent 1,212,758, to Amicon Corp., water is listed as a convenient gelling agent and PVDF is listed as candidate material for phase-inversion membranes but no examples are given for forming these PVDF membranes and moreover, no consideration is given to the problems of severe shrinkage when PVDF membranes are fabricated in aqueous gelling media.
The membranes produced by the several above-mentioned methods are flat, unwrinkled and uniform. However, scanning electron microscopy shows that the pore openings are non-discrete and the surface structure of the membrane is sponge-like. The spongy surface tends to trap and retain material when the membrane is used in filtration operations and thus the methane eventually becomes clogged with little prospect of clearing the entrapped material from the membrane matrix.
Polyvinylidene membranes of the present invention, by contrast, have discrete, undistorted pore openings in an undisturbed smooth surface. To achieve this, the phase inversion process is conducted in aqueous media and the PVDF layer is firmly anchored during its formation to the surface of the prefabricated microporous support. This not only prevents shrinkage and distortion of the PVDF layer in the course of its formation but strengthens the final membrane. Surprisingly, this lamination process occurs without adhesives, bonding agents or chemical reaction.
Lamination of membranes in series i.e., with surface-to-surface contact, has been widely used to reinforce fragile, thin membranes. In particular, lamination of membranes has been used to provide support for reverse osmosis (RO) membranes which must necessarily be thin in order to function efficiently but must also be able to withstand pressures of 100-800 psi. The most widely used support for these thin membranes are microporous polysulfone membranes having an average pore diameter of at least 0.1 .mu.m, although a variety of materials have been used to support such membranes, as disclosed in U.S. Pat. No. 4,848,700 to Fibiger, et al. For example, in U.S. Pat. No. 4,260,652, it is suggested that microporous PVDF membranes may serve as support structures for reverse osmosis membranes. Reverse osmosis membranes are generally characterized by pore sizes of less than 0.01 .mu.m.
It is accordingly an object of the present invention to provide a microporous PVDF series laminated composite membrane having discrete pore openings in an undistorted PVDF surface and which is useful in a wide variety of processes and under a variety of conditions.
It is another object of the present invention to provide a microporous PVDF composite membrane impregnated with a hydrophilic agent to adjust the hydrophobic/hydrophilic characteristics of the membrane.
Another object of the present invention is to provide a microporous PVDF composite membrane impregnated with specific concentrations of poly(2-hydroxyethyl methacrylate) (PHEMA) resulting in an alteration of the hydrophobicity/hydrophilicity of the membrane and an alteration in the permeability properties of the membrane.
It is another object of the present invention to provide a microporous composite PVDF membrane having a multiplicity of discrete pore openings at the exposed PVDF surface in the range of greater than 0.01 .mu.m up to about 0.2 .mu.m in diameter and which includes at least two membranes laminated in series wherein at least one membrane is comprised of polyvinylidene difluoride and an adjacent membrane is a supporting membrane.
A further object of the present invention is to provide a microporous composite PVDF membrane impregnated with a specific concentration of PHEMA such that the permeability properties of the membrane reflect those of PHEMA.
Still yet another object of the invention is to provide a composite microporous PVDF supported membrane useful for separating one component or group of components from a mixture of components in an aqueous or nonaqueous medium.
It is also an object of the invention to provide a composite microporous PVDF supported membrane which can be used to separate microorganisms or proteinaceous substances from a solution or suspension thereof.
A still further object of the invention is to provide a simple and economical process for preparing the aforesaid microporous PVDF supported composite membrane having discrete pore openings in the PVDF surface.